1. Field of the Invention
The invention in general relates to the structure and fabrication of electrical devices such as integrated circuits and more particularly to a new class of materials that provide low fatigue ferroelectric devices, reliable high dielectric constant devices, and are resistant generally to degradation in electronic applications.
2. Statement of the Problem
It has been known for many years that ferro-electric materials potentially offer significant advantages in integrated circuits, particularly integrated circuit memories. For example, the lowest cost, highest capacity integrated circuit memories, including conventional DRAMs, are volatile memories, that is, information stored in the memories remains only so long as power is applied to the integrated circuit. Currently available non-volatile memories, such as EPROMS or flash-type memories, are relatively costly, have relatively low storage density, require extremely high voltage applied for long periods to write and erase data, and generally have a more limited erase and write lifetime than DRAMs. It has long been recognized that ferroelectric materials have polarization states that can be selected or switched by application of an electric field, and that these polarization states remain after the electric field is removed. It is well-known that if a ferroelectric capacitor is substituted for the conventional silicon dioxide dielectric capacitor in the DRAM, instead of simply storing a charge that leaks off quickly, the capacitor can be switched between selected polarization states that will remain indefinitely after power is removed. Thus ferroelectric materials offer the possibility of simple, low cost, high density, non-volatile memories. Further, many semiconductor materials and devices, and in particular the low cost, high capacity memories such as conventional DRAMs, are susceptible to damage or alteration of their states from radiation. It is well-known that ferroelectric materials are highly resistant to radiation damage and that their ferroelectric states are highly resistant to being altered by radiation. Also, ferroelectric memories do not need high voltage for writing or erasing, and can be written to or erased as fast as conventional memories can be read. Thus, considerable research and development has been directed toward the design and manufacture of an integrated circuit memory utilizing the switchable property of ferroelectric materials. Such memories are described in U.S. Pat. No. 2,695,396 issued to Anderson, U.S. Pat. No. 4,144,591 issued to Brody, U.S. Pat. No. 4,149,301 issued to Cook, U.S. Pat. No. 4,360,896 issued to Brody, U.S. Pat. No. 5,046,043 issued to Miller et al., and Japanese Patent No. 2-304796. U.S. Pat. No. 5,028,455 issued to William D. Miller et al. and the article "Process Optimization and Characterization of Device Worthy Sol-Gel Based PZT for Ferroelectric Memories", in Ferroelectrics, Vol 109, pp. 1-23 (1990) summarize the various ferroelectric materials utilized in non-volatile memories. These materials include potassium nitrate (KNO.sub.3 ), PLZT (PbLa.sub.2 O.sub.3 -ZrO.sub.2 -TiO.sub.2), lead zirconate titanate or PZT (PbTiO.sub.3 -PbZrO.sub.3), and lead titanate (PbTiO.sub.3), with PZT currently being the material of choice. These ferroelectric materials are all random solid solutions meeting the general perovskite formula ABO.sub.3. Despite the considerable research and development efforts in ferroelectrics over the last thirty years, only a few specialty ferroelectric integrated circuits have been commercially produced.
A key problem in producing a non-volatile ferroelectric memory has been that up to now every ferroelectric material that has had acceptably high polarizability to be useful for a non-volatile memory has also been characterized by a high fatigue rate. That is, under repeated switching, the polarizability gradually decreases to less than 50% of the original polarizabilty. See the Ferroelectrics article referenced above. Moreover, when a series of pulses of the same sign are applied to the prior art materials, they take a set, or an imprint. See for example the article "Anomalous Remanent Polarization In Ferroelectric Capacitors" by Norman E. Abt, Reza Maazzami, and Yoav Nissan-Cohen, in Integrated Ferroelectrics, 1992, Vol 2, pp. 121-131, in which the phenomenon is referred to as .DELTA.P0. In terms of the hysteresis loop, the result is a shift in the loop in the direction of the field of the pulse. The effect of this imprinting can perhaps best be understood by considering the effect of point defects creating an internal bias field in the material. A ferroelectric is characterized by an energy diagram having relatively deep double wells, with a relatively low barrier between them. The wells represent the energy in the two polarization states. The wells in the virgin material have the same depth. Imprinting causes at least one well to deepen because of preferred relaxation in the direction of the internal bias field. The energy barrier from the shallower well to the deeper one becomes significantly smaller, and the barrier between the deeper well and the shallower one becomes significantly higher. As a result, when a field is applied to switch the material from the deeper state to the shallower state, it takes a longer time for the material to respond. Moreover, when the material is switched to the polarization state corresponding to the shallower well, the material will relax back into the deeper well state. At worst this causes the material to take a set so that it cannot function as a memory. At best, the timing of the memory must be slowed down considerably to account for the longest switching time.
A further problem with prior art ferroelectric materials is that the effective polarizability changes dramatically with the thickness of the material, generally decreasing as the material becomes thinner. This is believed to be due to surface effects. That is, the dipoles that create the ferroelectric properties are relatively uniformly distributed and continuous throughout the material, and therefore compensating charges occur predominately at the discontinuities, i.e. at the surface. A large surface charge is created in the prior art materials by the polarizing field, and most of the change in potential occurs at the surface of the material. This causes the bulk of the material to see a relatively small field. As the material becomes thinner, the surface effects overwhelm the bulk effects, and the material behaves more like a dielectric material than a ferroelectric material. This problem makes it difficult or impossible to make practical integrated circuits with the prior art materials, because integrated circuits are fabricated using thin films. Another problem has been that the ferroelectric materials tend to be incompatible with the semiconductor materials and structures used in integrated circuits. When the ferroelectric materials are combined with the conventional materials in conventional integrated circuit structures, they tend to interdiffuse with the other materials which either damages the ferroelectrics and other materials, alters their properties, or both. The ferroelectric materials also tend to lose their ferroelectric properties at the relatively sharp bends utilized in conventional memory device fabrication. Thus there remains a need for a ferroelectric integrated circuit structure and fabrication method that results in reliable, low cost, high density integrated circuit devices.
Aside from the non-volatile memory aspect discussed above, there has also been a need for a high dielectric constant material suitable for use in integrated circuits. The most commonly used dielectric material in integrated circuits is silicon dioxide, which has a dielectric constant of about 4. Capacitors using such a material must have a large area in order to provide the capacitive values required in state-of-the-art integrated circuits. These large areas make it difficult to reach high densities of capacitive components in an integrated circuit. However, the use of other materials to provide the dielectric in integrated circuits has been hindered by many of the same problems as ferroelectric memories: leakage of the dielectrics in the integrated circuit environment, degradation and breakdown of the materials caused by the stresses of fabrication and use over long time periods, and incompatibility of the materials with other common integrated circuit materials. As integrated circuits become smaller, this lack of a suitable high dielectric constant material becomes more and more significant. It is considered to be one of the serious roadblocks to 64 megabit and higher integrated circuit memories.
Up to now, the sciences of ferroelectrics and integrated circuit dielectric materials has been phenomenological. Researchers have made and tested a large number of materials and those which are found to be ferroelectric or to have high dielectric constants have been included in lengthy lists. See for example, Appendix F of Principles and Applications of Ferroelectrics and Related Materials, by M. E. Lines and A. M. Glass, Clarendon Press, Oxford, 1977, pp. 620-632. Generally the ferroelectrics in these lists have been separated into several classes such as the complex salts, such as KD.sub.2 PO.sub.4, the TGS type, such as tri-glycine sulfate, the perovskites, such as BaTiO.sub.3, SrTiO.sub.3 and PZT, or the tungsten bronzes, etc. The various classes a particular author chooses depends on his or her point of view. One class of ferroelectrics discovered by G. A. Smolenskii, V. A. Isupov, and A. I. Agranovskaya had a layered Perovskite structure. See Chapter 15 of the book, Ferroelectrics and Related Materials, ISSN 0275-9608, (V.3 of the series Ferroelectrics and Related Phenomena, 1984) edited by G. A. Smolenskii, especially sections 15.3-15.7; G. A. Smolenskii, A. I. Agranovskaya, "Dielectric Polarization of a Number of Complex Compounds", Fizika Tverdogo Tela, V. 1, No. 10, pp. 1562-1572 (October 1959); G. A. Smolenskii, A. I. Agranovskaya, V. A. Isupov, "New Ferroelectrics of Complex Composition", Soviet Physics--Technical Physics, 907-908 (1959); G. A. Smolenskii, V. A. Isupov, A. I. Agranovskaya, "Ferroelectrics of the Oxygen-Octahedral Type With Layered Structure", Soviet Physics--Solid State, V. 3, No. 3, pp. 651-655 (September 1961); E. C. Subbarao, "Ferroelectricity in Mixed Bismuth Oxides With Layer-Type Structure", J. Chem. Physics, V. 34, 695 (1961); E. C. Subbarao, "A Family of Ferroelectric Bismuth Compounds", J. Phys. Chem. Solids, V. 23, pp. 665-676 (1962) and Chapter 8 pages 241-292 and pages 624 & 625 of Appendix F of the Lines and Glass reference cited above. As outlined in section 15.3 of the Smolenskii book, the layered perovskite-like materials can be classified under three general types:
(I) compounds having the formula A.sub.m-1 Bi.sub.2 M.sub.m O.sub.3m+3, where A=Bi.sup.3+, Ba.sup.2+, Sr.sup.2+, Ca.sup.2+, Pb.sup.2+, K.sup.+, Na.sup.+ and other ions of comparable size, and M=Ti.sup.4+, Nb.sup.5+, Ta.sup.5+, Mo.sup.6+, W.sup.6+, Fe.sup.3+ and other ions that occupy oxygen octahedra; this group includes bismuth titanate, Bi.sub.4 Ti.sub.3 O.sub.12 ;
(II) compounds having the formula A.sub.m+1 M.sub.m O.sub.3m+1, including compounds such as strontium titanates Sr.sub.2 TiO.sub.4, Sr.sub.3 Ti.sub.2 O.sub.7 and Sr.sub.4 Ti.sub.3 O.sub.10 ; and
(III) compounds having the formula A.sub.m M.sub.m O.sub.3m+2, including compounds such as Sr.sub.2 Nb.sub.2 O.sub.7, La.sub.2 Ti.sub.2 O.sub.7, Sr.sub.5 TiNb.sub.4 O.sub.17, and Sr.sub.6 Ti.sub.2 Nb.sub.4 O.sub.20.
Smolenskii pointed out that the perovskite-like layers may have different thicknesses, depending on the value of m, and that the perovskite AMO.sub.3 is in principal the limiting example of any type of layered perovskite-like structure with m =infinity. Smolenskii also noted that if the layer with minimum thickness (m=1) is denoted by P and the bismuth-oxygen layer is denoted by B, then the type I compounds may be described as . . . BP.sub.m BP.sub.m . . . . Further Smolenskii noted that if m is a fractional number then the lattice contains perovskite-like layers of various thicknesses, and that all the known type I compounds are ferroelectrics. Similarly, Smolenskii noted that the type two compounds could be represented as . . . SP.sub.m SP.sub.m . . . where P is the perovskite-like layer of thickness m and S is the strontium-oxygen connecting layer, and that since the type I and type II compounds have similar perovskite-like layers, the existence of "hybrid" compounds such as . . . BP.sub.m SP.sub.n BP.sub.m SP.sub.m . . . "should not be ruled out", though none had been obtained at that time.
Up to now, these layered ferroelectric materials have not been considered as being suitable for non-volatile ferroelectric memories, nor have they been recognized as useful high dielectric constant materials. Two of the layered ferroelectric materials, bismuth titanate (Bi.sub.4 Ti.sub.3 O.sub.12) and barium magnesium fluoride (BaMgF.sub.4), have been used in a switching memory application as a gate on a transistor. See "A New Ferroelectric Memory Device, Metal-Ferroelectric-Semiconductor Transistor", by Shu-Yau Wu, IEEE Transactions On Electron Devices, Aug. 1974, pp. 499-504, which relates to the Bi.sub.4 Ti.sub.3 O.sub.12 device, and the article "Integrated Ferroelectrics" by J. F. Scott, C. A. Paz De Araujo, and L. D. McMillian in Condensed Matter News, Vol. 1, No. 3, 1992, pp. 16-20, which article is not prior art to this disclosure but Section IIB discusses experiments with ferroelectric field effect transistors (FEFETs). However, neither of these devices have been successfully used in a memory. In the case of the Bi.sub.4 Ti.sub.3 O.sub.12 device, the ON state decayed logarithmically after only two hours, and in the case of the BaMgF.sub.4 device, both states decayed exponentially after a few minutes. See "Memory Retention And Switching Behavior Of Metal-Ferroelectric-Semiconductor Transistors", by S. Y. Wu, Ferroelectrics, 1976 Vol. 11, pp. 379-383. Further, it is well known that at annealing temperatures, the titanium in Bi.sub.4 Ti.sub.3 O.sub.12 will react with the silicon in the substrate to form titanium silicide, a conductor, which will degrade the device beyond usefulness. Since without annealing Bi.sub.4 Ti.sub.3 O.sub.12 is not stable, and further since many of the materials in conventional integrated circuit require annealing for stability, the prior art suggests that useful memories cannot be made with Bi.sub.4 Ti.sub.3 O.sub.12. In the case of the BaMgF.sub.4 devices, the "polarized" state dissipates so rapidly that it is believed that the BaMgF.sub.4 is acting as an electret rather than a ferroelectric; that is, it is taking on a charged state and the state decays because the charge leaks off. Thus the prior art suggests that these materials cannot be used in a practical integrated circuit.
3. Solution to the problem
The invention solves the above problems by recognizing that a class of materials, called layered superlattice materials herein, are useful in integrated circuits. In particular the materials are highly useful as ferroelectric switching materials and as high dielectric constant materials.
The inventors have discovered that the intermediate layers between the ferroelectric layers in a layered superlattice material provide a "shock absorber" effect. This shock absorber effect prevents degradation of the ferroelectric material by repetitive switching. As a result the materials essentially do not fatigue and do not take an imprint. Further, the layered structure separates the dipoles that create the ferroelectric properties of the into very small domains, each of which has only a tiny space charge associated with it. As a result the surface effects do not occur, and the polarizability of these materials hardly varies with film thickness.
The shock absorber effect also prevents degradation or leaking of high dielectric constant materials.
Moreover, it has been discovered that the layered superlattice structure supports and binds the materials into desirable compounds that would not otherwise be stable without the superlattice. That is, without the superlattice, a desirable compound will degenerate into another less desirable compound.
As a result of the above features, the layered superlattice material lends itself to the creation of stable new materials and integrated circuit structures.
Many of these materials are compatible with conventional integrated circuit materials. For those that are not compatible, such as Bi.sub.4 Ti.sub.3 O.sub.12, the invention provides buffer layers which prevent diffusion and other material-degrading and electrical property degrading reactions from occurring.
While the layered superlattice materials have not up to now been recognized as being useful in integrated circuits, we have discovered that they are much more useful than any prior ferroelectric and high dielectric constant materials. Moreover, the shock absorber effect of the layered structure prevents the degradation of these properties by integrated circuit fabrication processes and structures, and the repetitive use common for integrated circuits. Thus reliable, durable materials having many desirable properties for integrated circuits and other electrical uses can be fabricated.
The problem of fatigue in ferroelectric switching devices is solved by utilizing a layered superlattice material as the ferroelectric switching material. The problem of fatigue in ferroelectric memories is solved by using a layered superlattice material in the storage cell of the memory. For example, strontium bismuth tantalate (SrBi.sub.2 Ta.sub.2 O.sub.9) has been made in thin films suitable for use in integrated circuits and having a polarizability, 2 Pr, of 25 microcoulombs/cm.sup.2 and showing less than 5% fatigue after 10.sup.10 cycles, which is equivalent to 10 years use in a typical integrated circuit switching device.
The problem of the need for a high dielectric constant material has been solved by utilizing a layered superlattice material as the dielectric in an integrated circuit. As an example, barium bismuth tantalate (BaBi.sub.2 Ta.sub.2 O.sub.9) having a dielectric constant of 166, measured at 1 Megahertz with a V.sub.osc of 15 millivolts, and a leakage current of 7.84.times.10.sup.-8 amps/cm.sup.2 at a field of 200 KV/cm has been made in thin films of less than 2000 .ANG., which are suitable for use in integrated circuits.
The solution of the above problems leads to other applications for the materials. Now that the function of the layered superlattice structure in providing a shock absorber effect which leads to lower fatigue and greater durability in ferroelectric and dielectric applications is understood, the principal can be applied to many other applications. As used herein, the term "low fatigue" as applied to a device or material capable of being switched from a first state to a second state means a device or material capable of switching 10.sup.9 times with less than 30% fatigue.